<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<h1>0</h1>
<h2>0</h2>
<script>
    let count = 0;
    //开启定时器,每隔一秒调用一次方法
    //方法作为参数传递时,不写()
    setInterval(f, 1000);
    function f() {
        count++;
        let h = document.querySelector("h1");
        h.innerText = count;
    }

    let num = 0;
    //匿名方法开启定时器
    let timer = setInterval(function(){
        num++;
        let h = document.querySelector("h2");
        h.innerText = num;
        if(num == 50){
            clearInterval(timer);
        }
    },100);
    //开启只执行一次的定时器
    setTimeout(function (){
        alert("时间到!")
    },3000)
</script>
</body>
</html>